Problem: 5 people can paint 6 walls in 45 minutes. How many minutes will it take for 10 people to paint 8 walls? Round to the nearest minute.
Explanation: We know the following about the number of walls $w$ painted by $p$ people in $t$ minutes at a constant rate $r$ $w = r \cdot t \cdot p$ $\begin{align*}w &= 6\text{ walls}\\ p &= 5\text{ people}\\ t &= 45\text{ minutes}\end{align*}$ Substituting known values and solving for $r$ $r = \dfrac{w}{t \cdot p}= \dfrac{6}{45 \cdot 5} = \dfrac{2}{75}\text{ walls painted per minute per person}$ We can now calculate the amount of time to paint 8 walls with 10 people. $t = \dfrac{w}{r \cdot p} = \dfrac{8}{\dfrac{2}{75} \cdot 10} = \dfrac{8}{\dfrac{4}{15}} = 30\text{ minutes}$